1. Field of the Invention
This invention relates to a variable time constant circuit, such as a variable time constant integrator or a variable cut-off frequency filter, and more particularly to a variable time constant circuit using a field effect transistor and a filter circuit using the variable time constant circuit.
2. Description of the Related Art
To make a transfer characteristic of an active filter, such as a cut-off frequency, variable, the time constant of a time constant circuit included in the active filter has to be changed. One known method for achieving this is to use the drain-source resistance (on resistance) of a field effect transistor in the triode region as a variable resistor for a variable time constant circuit to control the gate voltage of the transistor, thereby changing the time constant. For example, U.S. Pat. No. 4,710,726 has disclosed a variable resistor circuit network that cancels odd-degree distortions by using a plurality of FETs (field effect transistors) differing in gate voltage.
In a circuit using the on resistance of such an FET as a variable resistor, the drain-source of the FET is connected in series with a signal source. Thus, when the power supply voltage gets lower, the gate-source voltage of the FET cannot be made sufficiently high, with the result that the FET turns off. As described above, in the conventional variable time constant circuit, there is a possibility that the FET used as a variable resistor will not turn on at a low power supply voltage and therefore will not function as an integrator or a filter.
It is, accordingly, an object of the present invention to provide a variable time constant circuit capable of operating positively even at a low power supply voltage and a filter circuit using the viable time constant circuit.